In conventional plastic injection molding, the molten plastic material is injected through sprue bushings into the cavity of a hollow mold. Often, two or more sprue bushings attached to a common manifold are used. In this instance, the plastic melt coming from the injection molding machine through runners is distributed by a heated distributor block (a/k/a manifold block) to the individual sprue bushings. The sprue bushings are positioned in openings of a nozzle plate which typically forms the upper half of a two-part mold. The injection mold can accommodate several cavities of the same or of different sizes, each of which is connected to a sprue bushing. Also, in the case of large work pieces, two or more sprue bushings may be connected with the same cavity in a mold.
One conventional method used to control the melt flow utilizes one or more needle valves, also known as valve gate bushings. With these systems, the needle valve is inserted through the distributor block or manifold into the sprue bushings and is controlled for axial movement by a control device or mechanism. The needle valve has an elongated pin which is moved axially by the control device or mechanism and is adapted to fit within an orifice in the end of the sprue bushing in order to open and close the passageway of plastic melt from the sprue bushing into the mold cavity. The control mechanism operates either hydraulically or pneumatically. When manufacturing certain plastic products, such as medical products, hydraulic control devices should not be used since any possibility of contact between the injection melt and the final products with oil must be avoided.
During the injection molding process, the manifold (distributor block) heats up to a higher temperature than the control mechanism and its mounting. Due to thermal expansion, the sides of the distributor block move or shift laterally relative to the control mechanism. This causes lateral forces to act on the control piston, which in turn could lead to sealing problems between the control piston and the cylinder wall. Also, due to manufacturing tolerances, alignment errors may occur between the control piston axis and the longitudinal axis of the valve needle port in the distributor block that can also lead to an irregular contact pressure of the seal between the control piston and control cylinder.
Aside from impairing and damaging the seal, the needle also may become bent. The needle has a snug fit inside the seal and sprue bushing and is sensitive to lateral forces. During the injection process, the needle or pin moves twice axially within a relatively short period of time, e.g. 20-30 seconds, in order to open and close the valve. The seal bushing also has a highly sensitive coating in order to obtain the requisite sealing power.
Another disadvantage of conventional control devices, in which the operating fluid for the piston is supplied radially through the wall of the control cylinder, is the fact that several seals are required between the exterior wall of the cylinder mantle and the cylinder receptacle. Installing the control cylinder with these seals in the cylinder receptacle is difficult and care must be taken that the seals do not become damaged during installation. To facilitate installation, it is common to design the exterior cylinder wall in several graduated layers in the axial direction. However, this results in higher manufacturing costs for the system. This disadvantage exists whether or not the needle valve passes through a manifold and whether or not alignment errors lead to lateral forces as described above.